This invention pertains generally to intrusion alarm systems and more particularly to a system in which the presence of an intruder is detected by infrared heat energy emitted by his body.
Infrared intrusion alarms systems heretofore provided generally utilize means including a sensing element for producing an electrical signal corresponding to the level of infrared energy received from an area to be protected. The signal is processed by suitable circuitry, and an alarm is actuated in the event of an abrupt change in the signal, as occurs when a warm-bodied intruder enters the protected area. Systems of this type are described in U.S. Pat. Nos. 3,703,718 and 3,928,843.
Such systems depend for their operation upon the difference or contrast in level between the radiation emitted by an intruder and the radiation produced by background objects which are normally present in the protected area, and the sensitivity or detection range of such systems is therefore dependent upon ambient temperature. For the small temperature differences which normally exist between the intruder and the background objects, the contrast signal is very nearly proportional to the difference in temperature between the intruder and the background. As the temperature of the protected area increases and approaches the temperature of the intruder, the contrast signal decreases, and the detection range is reduced. Conversely, when the temperature of the protected area decreases, the contrast signal increases, and the detection range also increases. However, if the background temperature is lowered significantly, the detection system may be subject to false alarms from spurious thermal sources within the protected area.